The present invention relates to lift trucks generally and more specifically to a suspension system which isolates a vertically raisable lift carriage from other truck components.
Most lift trucks include a lift carriage mounted to a mast which is in turn mounted to a tractor. The tractor includes a plurality of wheels which facilitate horizontal truck movement within the factory, warehouse, or the like. The mast includes a mainframe attached to the tractor and may include one or more telescopics. Generally, trucks that service shelves at greater heights will use one, two and sometime three telescopics to extend the maximum elevated fork height without substantially increasing the fully lowered height of the truck. In all cases, the carriage is mounted to the innermost telescopic. The mast also includes one or more ram/chain mechanisms which facilitate vertical movement of the carriage. Typically the carriage will be mounted to the track of the mainframe or telescopic for movement therealong. The lower end of the ram will be mounted in a fixed position to the mainframe or telescopic. The ram includes a pulley mechanism at its upper end. A chain connected at a first end to an anchor which is fixed in a single position with respect to the mainframe or telescopic, extends upwardly over the pulley mechanism and is connected to the carriage at a second end. To raise the carriage with the load, the ram is extended. Because the first end of the chain is fixed, when the ram is extended, the chain's second end is raised, lifting the carriage.
If the vehicle contains no telescopics, the carriage will be mounted to the track of the mainframe for movement therealong and the base of the ram will be mounted to the mainframe. If the vehicle contains a single telescopic, the pulley mechanism described above will also be attached to the telescopic for vertical movement thereof. The carriage will be mounted to the track of the telescopic for movement therealong and the telescopic will be mounted to the track of the mainframe for movement therealong. The base of the ram and the first end of the chain will be fixed to the mainframe. If the vehicle includes two telescopics, the first telescopic is typically raised in a manner similar to the above using one or more additional ram/chain mechanisms.
In this case the ram(s) which elevates the carriage is fixed to the telescopic to which the carriage is mounted.
In all of the above configurations, the dimensions of the components are chosen so that throughout the full range of vertical motion of the carriage, including the fully lowered position, the total carriage weight is suspended by the aforementioned ram/chain mechanism.
In modern lift truck applications it is desirable to elevate to increasingly greater heights. As is well known in the industry, a rigid mast and tractor are desirable to retain stability at the greater heights. Unfortunately, a stiff mast and tractor system can permit the transmission of severe vibrations and oscillations to the carriage. This is because the truck described above does not isolate the carriage from truck vibrations which, in many cases, are magnified as they are transmitted through the truck. This later phenomenon is particularly true where the vibrations are at the same frequency as a natural oscillating frequency of the truck.
One common carriage attachment is a lift fork including two or more horizontal lifting arms. The arms or forks can be slid under a load and raised via the carriage. In this case if the carriage vibrations are sufficient, a load on the forks can shift. A shifted load can at least contribute to a perception of instability which will cause an operator to slow the operation of the truck, thereby reducing the overall productivity.
Another common carriage attachment is an operator's carriage. For this reason, vibrations are often transmitted to the operator's carriage and tend to cause operator discomfort.
Moreover, because the ram/chain mechanism is rigid, the ram/chain components are subjected to extreme stress each time the truck is used which reduces the useful life of the components.
The industry has generally recognized operator discomfort and load carrying problems due to truck vibrations during operation and has attempted to solve these problems in a number of different ways. One solution has been to provide a better wheel suspension system. Unfortunately, better suspension systems can further decrease truck stability. For example, wheel deflections can cause a truck to "rock" laterally. This is particularly problematic when a load is suspended at extended elevated heights or when the truck is operating in a very narrow aisle, which is often the case.
Another solution is described in U.S. Pat. No. 3,574,383 which teaches a leaf spring mounted fork, the fork freely and pivotally suspended from a single central spring section to permit "lateral sway" and "lateral resiliency". Unfortunately, while lateral sway may be important in the environment contemplated by the '383 system (i.e. severely uneven terrain in the lateral dimension where one of more wheels may be independently lifted off the ground and the load is relatively low at all times), in the present case, lateral sway cannot be tolerated. In the warehouse environment where aisles are narrow, tractor wheels are relatively close together and the carriage is often disposed along an upper section of the mast, lateral sway causes lateral truck instability which can result in collisions between the carriage and warehouse fixtures. In addition, the '383 system is relatively complex and there fore would be expensive to manufacture.
Another solution has been to provide foam or rubber floor mats inside an operator's station. This solution, however, can only isolate an operator from high frequency vibrations and, it has been found, can exacerbate transmission of low frequency vibrations. In addition, this solution does not eliminate or reduce vibration transmission to the fork and load.
Yet one other solution has been to mechanically isolate the operator platform floor. Unfortunately, this solution also has a number of shortcomings. Once again, solutions of this type are typically expensive. In addition, to effectively isolate an operator in this manner requires a substantial suspended floor which can result in relative motion between an operator and truck controls. Moreover, even if a suspended floor can be designed which effectively isolates an operator from truck vibrations, such isolation does not eliminate load and fork vibrations.
Therefore, it would be advantageous to have a system for use with a lift truck which isolates a lift carriage from tractor vibrations which is simple to manufacture, inexpensive, stable, and durable.